1. Field of the Invention
This invention relates to an automatic gain control apparatus in an NMR imaging system.
2. Description of Prior Art
NMR (nuclear magnetic resonance) imaging systems are known. One method of measuring an NMR signal is the multi-echo method. An example of the multi-echo method is shown in FIG. 4, which comprises lines (A)-(E), and which depicts the pulse sequence used in the multi-echo method. A two-dimensional multi-echo method involves the following steps. As shown in FIG. 4, a static field is imparted to the body or object being examined. Concurrently a Z gradient magnetic field G.sub.Z - (line B) is imparted to the body. At the same time, a 90.degree. RF pulse (line A) is impressed on the body for selective excitation. Then, gradient magnetic field G.sub.Z + (line B) is impressed for only a period of time t.sub.Z2. At the same time, an X gradient magnetic field G.sub.X (line C) and a Y gradient magnetic Field G.sub.Y (line D) are impressed. In this case, G.sub.Z is equal to EQU g.sub.Z1 .multidot.t.sub.Z1 =g.sub.Z2 .multidot.t.sub.Z2
wherein,
g.sub.Z1 is the strength of G.sub.Z.sup.- PA1 t.sub.Z1 is the impressing time of G.sub.Z1 PA1 g.sub.Z2 is the strength of G.sub.Z.sup.+ PA1 t.sub.Z2 is the impressing time of G.sub.Z2 and PA1 G.sub.Y is equal to EQU G.sub.Yo X n PA1 g.sub.X1 and g.sub.X2 are the strength of G.sub.X PA1 t.sub.X1 is the impressing time of G.sub.X1 PA1 t.sub.X2 is the impressing time of G.sub.X2.
wherein n is the view number, which varies as follows -N, -N, +1, . . . -1,0, 1,2, . . . +N.
After time .tau. (line A) from the impression of the 90.degree. pulse, a 180.degree. pulse is impressed for turning spins in dispersion through an angle of 180.degree.. At that time, all gradient magnetic fields are removed.
Subsequently, X gradient magnetic field G.sub.X is impressed (line C). A first echo signal (line E) is generated, which becomes maximum after time .tau. from the point of impression of the 180.degree. pulse. In this case, in the impression of the X gradient magnetic field, EQU g.sub.X1 19 t.sub.X1 =g.sub.X2 .multidot.t.sub.X2
wherein
In the next step, after time .tau. from the starting point, which is at the time point when the first echo signal is peaked, a second 180.degree. pulse (line A) is impressed. During the observation of the first echo signal, an X gradient magnetic field (line C) is continuously impressed. Then, after time .tau. from the impression of the second 180.degree. pulse, a third 180.degree. pulse (line A) is impressed. In this case, the impression of the X gradient magnetic field takes place after termination of the second 180.degree. pulse at the strength of magnetic field of g.sub.X2 for a period of time t.sub.X2 (line C).
Similarly, to the effect of the first 180.degree. pulse, the second 180.degree. pulse acts to invert the spin to produce the second echo signal (line E). Thus in the same manner, using the third 180.degree. pulse, etc, and g.sub.xn magnetic field, a plurality of echo signals (third, etc.) are obtained with respect to the view.
As for other views, a multiplicity of echoes can be observed by the same operation, with gradual varying of the strength of the Y gradient magnetic field G.sub.Y. Thus, a multiplicity of echo signals are produced by impressing a 180.degree. pulse for each view. These echo signals are then observed.
However, in the multi-echo method, because analog to digital conversion (called A/D conversion) of echo signals (see phantom line (dotted line) of FIG. 4, line E) there is a tendency for the resultant echo signals to attenuate chronologically. Thus, a wide dynamic range is required of the A/D converter to make the A/D conversion. Moreover, since reconstitution of the image takes place for each echo signal, the image of the first echo would be relatively bright (that is the signal strength is great), but the image of the second and subsequent echoes would be relatively increasingly darker. Thus, the images of the successive echoes become more difficult to observe on the same display.
One solution to the problem of decreasing quality of the successive images is to control the brightness adjustment of the display by dividing the region. But, this solution is unsatisfactory because it requires repeated and continuous manipulation of the controls.
Another solution is to compress the data logarithmically to provide a wide dynamic range. But, this solution is also unsatisfactory since the imaging system would be unduly complex.